I'll throw my anecdote into the mix:
When I installed the Spectre BIOS update, I immediately noticed that the overall responsiveness of my 5820K dropped further than I would have liked. The worst impact I saw besides what others reported was stuttering every few seconds of software decoded video streams, such as Amazon Prime in Firefox. GPU accelerated video (Amazon Prime in Microsoft Edge, Netflix Win 10 app) was unaffected.
I also noticed that switching browser tabs was perceptably lagging at times (while the video stream was running) when it never did before.
I reverted the BIOS update to the previous version and all the above symptoms went away. I'm going to leave the old BIOS until Intel releases their next version of the microcode. If it still kills my 5820, I'm hoping I can gain back some of the old performance by doing some light overclocking (assuming that will help at all).